1. Field of the Invention
This invention relates to an air-fuel ratio control system for internal combustion engines, and more particularly to an air-fuel ratio control system of this kind, which carries out feedback control of the air-fuel ratio of a mixture supplied to the engine, by applying an adaptive control theory thereto.
2. Prior Art
Conventionally, an air-fuel ratio control system for internal combustion engines is known, e.g. from Japanese Laid-Open Patent Publication (Kokai) No. 7-247886, which calculates an air-fuel ratio control amount by using an adaptive controller including a parameter-adjusting mechanism of a recurrence formula type based on an adaptive control theory, and carries out feedback control of the air-fuel ratio of a mixture supplied to the engine, based on the calculated air-fuel ratio control amount. In the known air-fuel ratio control system, an air-fuel ratio sensor arranged in the exhaust system of the engine detects the air-fuel ratio and supplies an output indicative of the detected air-fuel ratio to the adaptive controller, which carries out the air-fuel ratio feedback control in response to the detected air-fuel ratio.
On the other hand, so-called PID control is widely employed to calculate an air-fuel ratio control amount in response to the output from the air-fuel ratio sensor, to thereby feedback-control the air-fuel ratio of the mixture supplied to the engine. When the air-fuel ratio control amount thus calculated falls outside a range defined by predetermined upper and lower limit values due to a disturbance during execution of the air-fuel ratio feedback control, limit-checking of the air-fuel ratio control amount and its integral term is executed to hold the air-fuel ratio control amount at the predetermined upper and lower limit values and the integral term at predetermined upper and lower limit values, to avoid sharp fluctuations in the air-fuel ratio.
When such limit-checking is executed during the air-fuel ratio control using the adaptive controller, however, the integral term cannot be separated from the equation for calculating the air-fuel ratio control amount, in a simple manner, for limit-checking thereof. Therefore, for example, as shown in FIG. 1, when a large disturbance temporarily takes place during execution of the air-fuel ratio control over a time period between time points t1 and t2, the air-fuel ratio control amount KSTR per se can be limit-checked and held at a limit value thereof at and after a time point t3, but the integral term cannot be limit-checked. As a result, even after the disturbance blows out, the air-fuel ratio control amount KSTR is held at the limit value over a considerable time period, and therefore the KSTR value returns to a state assumed before the appearance of the disturbance only after the considerable time period has elapsed. This inconvenience is caused by the fact that parameters corresponding to the integral term of the air-fuel ratio control amount KSTR continue to increase until a time point t2 at which the disturbance blows out, as indicated by the solid line in FIG. 1.